573 research outputs found
Selmer Groups in Twist Families of Elliptic Curves
The aim of this article is to give some numerical data related to the order
of the Selmer groups in twist families of elliptic curves. To do this we assume
the Birch and Swinnerton-Dyer conjecture is true and we use a celebrated
theorem of Waldspurger to get a fast algorithm to compute . Having
an extensive amount of data we compare the distribution of the order of the
Selmer groups by functions of type with small. We discuss how the
"best choice" of is depending on the conductor of the chosen elliptic
curves and the congruence classes of twist factors.Comment: to appear in Quaestiones Mathematicae. 16 page
Air-snow transfer of nitrate on the East Antarctic plateau â Part 2: An isotopic model for the interpretation of deep ice-core records
Unraveling the modern budget of reactive nitrogen on the Antarctic plateau is critical for the interpretation of ice core records of nitrate. This requires accounting for nitrate recycling processes occurring in near surface snow and the overlying atmospheric boundary layer. Not only concentration measurements, but also isotopic ratios of nitrogen and oxygen in nitrate, provide constraints on the processes at play. However, due to the large number of intertwined chemical and physical phenomena involved, numerical modelling is required to test hypotheses in a~quantitative manner. Here we introduce the model "TRansfer of Atmospheric Nitrate Stable Isotopes To the Snow" (TRANSITS), a~novel conceptual, multi-layer and one-dimensional model representing the impact of processes operating on nitrate at the airâsnow interface on the East Antarctic plateau, in terms of concentrations (mass fraction) and the nitrogen (ÎŽ15N) and oxygen isotopic composition (17O}-excess, Î17O) in nitrate. At the airâsnow interface at Dome C (DC, 75°06' S, 123°19' E), the model reproduces well the values of ÎŽ15N in atmospheric and surface snow (skin layer) nitrate as well as in the ÎŽ15N profile in DC snow including the observed extraordinary high positive values (around +300 â°) below 20 \unit{cm}. The model also captures the observed variability in nitrate mass fraction in the snow. While oxygen data are qualitatively reproduced at the airâsnow interface at DC and in East Antarctica, the simulated Î17O values underestimate the observed Î17O values by a~few~â°. This is explained by the simplifications made in the description of the atmospheric cycling and oxidation of NO2. The model reproduces well the sensitivity of ÎŽ15N, Î17O and the apparent fractionation constants (15Ï”app, 17Eapp) to the snow accumulation rate. Building on this development, we propose a~framework for the interpretation of nitrate records measured from ice cores. Measurement of nitrate mass fractions and ÎŽ15N in the nitrate archived in an ice core, may be used to derive information about past variations in the total ozone column and/or the primary inputs of nitrate above Antarctica as well as in nitrate trapping efficiency (defined as the ratio between the archived nitrate flux and the primary nitrate input flux). The Î17O of nitrate could then be corrected from the impact of cage recombination effects associated with the photolysis of nitrate in snow. Past changes in the relative contributions of the Î17O in the primary inputs of nitrate and the Î17O in the locally cycled NO2 could then be determined. Therefore, information about the past variations in the local and long range processes operating on reactive nitrogen species could be obtained from ice cores collected in low accumulation regions such as the Antarctic plateau
Random Resistor-Diode Networks and the Crossover from Isotropic to Directed Percolation
By employing the methods of renormalized field theory we show that the
percolation behavior of random resistor-diode networks near the multicritical
line belongs to the universality class of isotropic percolation. We construct a
mesoscopic model from the general epidemic process by including a relevant
isotropy-breaking perturbation. We present a two-loop calculation of the
crossover exponent . Upon blending the -expansion result with
the exact value for one dimension by a rational approximation, we
obtain for two dimensions . This value is in agreement
with the recent simulations of a two-dimensional random diode network by Inui,
Kakuno, Tretyakov, Komatsu, and Kameoka, who found an order parameter exponent
different from those of isotropic and directed percolation.
Furthermore, we reconsider the theory of the full crossover from isotropic to
directed percolation by Frey, T\"{a}uber, and Schwabl and clear up some minor
shortcomings.Comment: 24 pages, 2 figure
The Fall of Stringy de Sitter
Kachru, Kallosh, Linde, & Trivedi recently constructed a four-dimensional de
Sitter compactification of IIB string theory, which they showed to be
metastable in agreement with general arguments about de Sitter spacetimes in
quantum gravity. In this paper, we describe how discrete flux choices lead to a
closely-spaced set of vacua and explore various decay channels. We find that in
many situations NS5-brane meditated decays which exchange NSNS 3-form flux for
D3-branes are comparatively very fast.Comment: 35 pp (11 pp appendices), 5 figures, v3. fixed minor typo
Evidence for a novel Carbohydrate Binding Module (CBM) of Tannerella forsythia NanH sialidase, key to interactions at the host-pathogen interface
Bacterial sialidases cleave terminal sialic acid from a variety of host glycoproteins, and contribute to survival and growth of many human-dwelling bacterial species, including various pathogens.Tannerella forsythia, an oral, Gram-negative, fastidious anaerobe, is a key organism in periodontal disease, and possesses a dedicated sialic acid utilisation and scavenging (nan) operon, including NanH sialidase. Here, we describe biochemical characterisation of recombinant NanH, including its action on host-relevant sialoglycans such as sialyl Lewis A and sialyl Lewis X (SLeA/X), and on human cell-attached sialic acids directly, uncovering that it is a highly active broad specificity sialidase. Furthermore, theN-terminal domain of NanH was hypothesised and proven to be capable of binding to a range of sialoglycans and non-sialylated derivatives withKdin the micromolar range, as determined by steady-state tryptophan fluorescence spectroscopy, but it has no catalytic activity in isolation from the active site. We consider this domain to represent the founding member of a novel subfamily of Carbohydrate Binding Module (CBM), involved in glycosidase-ligand binding. In addition, we created a catalytically inactive version of the NanH enzyme (FRIPâYMAP) that retained its ability to bind sialic acid-containing ligands and revealed for the first time that binding activity of a CBM is enhanced by association with the catalytic domain. Finally, we investigated the importance of Lewis-type sialoglycans onT. forsythia-host interactions, showing that nanomolar amounts of SLeA/Xwere capable of reducing invasion of oral epithelial cells byT. forsythiasuggesting that these are key ligands for bacterial-cellular interactions during periodontal disease
Interface Scaling in the Contact Process
Scaling properties of an interface representation of the critical contact
process are studied in dimensions 1 - 3. Simulations confirm the scaling
relation beta_W = 1 - theta between the interface-width growth exponent beta_W
and the exponent theta governing the decay of the order parameter. A scaling
property of the height distribution, which serves as the basis for this
relation, is also verified. The height-height correlation function shows clear
signs of anomalous scaling, in accord with Lopez' analysis [Phys. Rev. Lett.
83, 4594 (1999)], but no evidence of multiscaling.Comment: 10 pages, 9 figure
de Sitter String Vacua from Supersymmetric D-terms
We propose a new mechanism for obtaining de Sitter vacua in type IIB string
theory compactified on (orientifolded) Calabi-Yau manifolds similar to those
recently studied by Kachru, Kallosh, Linde and Trivedi (KKLT). dS vacuum
appears in KKLT model after uplifting an AdS vacuum by adding an anti-D3-brane,
which explicitly breaks supersymmetry. We accomplish the same goal by adding
fluxes of gauge fields within the D7-branes, which induce a D-term potential in
the effective 4D action. In this way we obtain dS space as a spontaneously
broken vacuum from a purely supersymmetric 4D action. We argue that our
approach can be directly extended to heterotic string vacua, with the dilaton
potential obtained from a combination of gaugino condensation and the D-terms
generated by anomalous U(1) gauge groups.Comment: 17 pages, 1 figur
Field Theory Approaches to Nonequilibrium Dynamics
It is explained how field-theoretic methods and the dynamic renormalisation
group (RG) can be applied to study the universal scaling properties of systems
that either undergo a continuous phase transition or display generic scale
invariance, both near and far from thermal equilibrium. Part 1 introduces the
response functional field theory representation of (nonlinear) Langevin
equations. The RG is employed to compute the scaling exponents for several
universality classes governing the critical dynamics near second-order phase
transitions in equilibrium. The effects of reversible mode-coupling terms,
quenching from random initial conditions to the critical point, and violating
the detailed balance constraints are briefly discussed. It is shown how the
same formalism can be applied to nonequilibrium systems such as driven
diffusive lattice gases. Part 2 describes how the master equation for
stochastic particle reaction processes can be mapped onto a field theory
action. The RG is then used to analyse simple diffusion-limited annihilation
reactions as well as generic continuous transitions from active to inactive,
absorbing states, which are characterised by the power laws of (critical)
directed percolation. Certain other important universality classes are
mentioned, and some open issues are listed.Comment: 54 pages, 9 figures, Lecture Notes for Luxembourg Summer School
"Ageing and the Glass Transition", submitted to Springer Lecture Notes in
Physics (www.springeronline/com/series/5304/
Measurement of the Charged Multiplicities in b, c and Light Quark Events from Z0 Decays
Average charged multiplicities have been measured separately in , and
light quark () events from decays measured in the SLD experiment.
Impact parameters of charged tracks were used to select enriched samples of
and light quark events, and reconstructed charmed mesons were used to select
quark events. We measured the charged multiplicities:
,
, from
which we derived the differences between the total average charged
multiplicities of or quark events and light quark events: and . We compared
these measurements with those at lower center-of-mass energies and with
perturbative QCD predictions. These combined results are in agreement with the
QCD expectations and disfavor the hypothesis of flavor-independent
fragmentation.Comment: 19 pages LaTex, 4 EPS figures, to appear in Physics Letters
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